(1) Field of the Invention
The present invention relates to non-aqueous electrolyte cells and is particularly concerned with sealed non-aqueous electrolyte cells having electric energy generating elements sealed within a casing that is formed from a laminated material made from a metal foil and a resin film.
(2) Description of the Prior Art
Heretofore, metal sheets such as stainless steel sheets (SUS) have been used as casing materials for sealed cells. However, such a metal sheet has high rigidity and therefore cannot be easily formed into an extremely thin flat shape. In addition, the metal sheet has high specific gravity which causes a decrease in energy density per unit weight of the cell when used in the fabrication of extremely thin cells.
For these reasons, lately developed sheet-like (i.e., extremely thin flat plate type) sealed non-aqueous electrolyte cells use, as a casing material, a laminated material comprising a metal foil laminated to a resin film. Such a laminated material is lightweight and flexible compared to stainless steel sheets etc., and therefore can be easily formed into an extremely thin flat shape. The laminated material can be fused by heat, which facilitates the sealing of cells. Additionally, it has a structure in which a metal foil layer having low gas permeability and a resin layer having excellent chemical stability are laminated together, and the former (i.e., the metal foil) functions to inhibit permeation of moisture, oxygen and others while the latter (i.e., the resin layer) functions as a protective layer against oxygen and alkali. With this structure, the laminated material, as a whole, excels in gas permeability as well as in chemical stability. Accordingly, the laminated material has been regarded as a useful material for forming a casing for sheet-like cells which accommodate highly reactive electric energy generating elements.
Despite the above advantages, there has not been developed yet a sheet-like sealed non-aqueous electrolyte cell which has a casing made from a laminated material and provides satisfactory service life, because the degree of sealing in the prior art electric cells decreases with time.
The present invention is directed to overcoming the above-described problem presented by the prior art sealed non-aqueous electrolyte cells having a casing made from a laminated material. One of the objects of the invention is therefore to provide a sheet-like sealed non-aqueous electrolyte cell having a laminated material casing, which cell includes a means for preventing a trace amount of water present therein from causing deterioration in the performance of the cell.
The above object can be achieved by a sealed non-aqueous electrolyte cell according to the invention having: a casing that is made from a laminated material comprising a metal foil and a resin film; and an inorganic oxide fine powder that is not an electrode active material and is accommodated within the casing together with electric energy generating elements.
Generally, sealed non-aqueous electrolyte cells are apt to be damaged by moisture but it is difficult to completely exclude moisture from the cells in the fabrication process. Therefore, a finished product often includes water which was originally contained, for example, in an organic solvent (a component of the electrolyte) or derived from moisture on the cell members. Water entrapped in the cell reacts with the electrolyte salt and the active material and becomes a direct cause of deterioration in the performance of the cell. Hydrofluoric acid or the like, which is a product of the reaction between cell components and water entrapped within the cell, acts on the sealing parts of the casing, decreasing adhesive strength and therefore the degree of sealing in the parts. This causes a vicious circle in which the decreased degree of sealing allows penetration of moisture and oxygen from the outside of the cell and the penetrating moisture causes a further decrease in the degree of sealing. Finally, there occurs leakage, resulting in the expiration of the cell""s life.
It is understood from the above that the decreased degree of sealing is a cause of shortening the service life of the prior art sheet-like sealed non-aqueous electrolyte cells.
As opposed to the prior art cell, the present invention having the above-described structure includes an inorganic oxide fine powder which is not an electrode active material and this fine powder adsorbs and inactivates a trace amount of water which has been present in the cell since completion of the fabrication or moisture which has penetrated into the cell after the fabrication. The fine powder also adsorbs hydrofluoric acid etc. produced by the reaction between the water within the cell and the cell components. Additionally, the inorganic oxide fine powder contained in the cell enhances the viscosity of the cell components thereby restricting scatter and evaporation of the cell components and the reaction products including hydrofluoric acid. With this arrangement, the extent to which hydrofluoric acid and others come in direct contact with the sealing parts of the casing can be reduced, which, in consequence, reduces the possibility of degradation in the sealing parts.
By virtue of the inorganic oxide fine powder favourably functioning as described above, it is possible to prevent a decrease in the performance of the cell directly caused by the reaction between the cell components and water and to prevent a decrease in the degree of sealing caused by the reaction products of the electrolyte salt etc. and water. This leads to an increase in the cycling capability of the cell. In short, the invention has the capability to prevent a decrease in the performance of a cell, which decrease is attributable to the reaction between water penetrated into the cell and the components of the cell.
The sealing parts defined herein are the parts where the opposed seal edges of the laminated material are bonded to each other to seal the cell. Bonding of these edges is usually achieved by thermal fusion but may be achieved by an adhesive agent, because the use of an adhesive agent does not spoil the effects of adding the inorganic oxide fine powder.
In the invention, the inorganic oxide fine powder may be contained in an electrolyte.
The electrolyte of a non-aqueous electrolyte cell is usually composed of an organic solvent and an electrolyte salt such as lithium salt, and the organic solvent contains trace amounts of water while the electrolyte salt is highly reactive with water. In cases where the electrolyte includes the inorganic oxide fine powder, the fine powder absorbs water contained in the organic solvent so that the hydrolysis reaction between the water in the organic solvent and the electrolyte salt can be restrained. Even if reaction products are produced to some extent by hydrolysis, the reaction products will be adsorbed by the inorganic oxide fine powder. In addition, the inorganic oxide fine powder contained in the electrolyte enhances the viscosity of the electrolyte so that scatter and evaporation of the components of the electrolyte and the reaction products can be restrained. Accordingly, when blended with the electrolyte, the inorganic oxide fine powder can work effectively to restrain a decrease in the degree of sealing.
In the invention, the inorganic oxide fine powder may have an average particle diameter of 5 xcexcm or less.
When the inorganic oxide fine powder has a particle diameter of 5 xcexcm or less, the total surface area of the fine powder is large enough to provide a strong power for adsorbing water, hydrofluoric acid and others and to favourably increase the viscosity of the electrolyte. This arrangement restrains the degree of sealing from decreasing and as a result, improved cycling capability can be ensured.
In the invention, the inorganic oxide fine powder may be contained in amounts of 0.05 to 20% by weight of the electrolyte.
When the amount of the inorganic oxide fine powder contained in the electrolyte is 0.05 wt % or more, the fine powder can satisfactorily adsorb water, hydrofluoric acid and others. Therefore, the degradation of the sealing parts of the casing can be more effectively restrained which ensures that leakage caused by a decrease in the degree of sealing is prevented. If the amount of the inorganic oxide fine powder exceeds 20 wt %, the fine powder causes an undesirable increase in the viscosity of the electrolyte, which entails a decrease in the discharge capacity of the cell. In addition, energy density decreases with the increasing amount of the inorganic oxide fine powder that is not an electrode active material. In order to improve the service life of the cell without impairing its discharge rate characteristic, the suitable amount of the inorganic oxide fine powder based on the weight of the electrolyte should be in the range of from 0.05 to 20 wt %.
The electrolyte in the invention may be a solid polymer electrolyte.
Since solid polymer electrolytes are inferior in ionic conductivity to liquid electrolytes, a preferred shape for the cell is a thin flat shape. The reason for this is that, when the cell is formed into a thin flat shape, the distance between the anode and cathode can be shortened, facilitating ionic conduction between the anode and cathode. In cases a rigid metal sheet such as stainless steel sheets is used as the material of a casing, the difficulty lies in forming such a sheet into a thin flat shape. Also, it is difficult to insert the electric energy generating elements into a rigid, extremely thin casing and therefore, when assembled into a cell, the electric energy generating elements easily get damaged so that a high quality cell is hardly obtained.
As opposed to cells having a metal casing, the present invention utilizes a soft laminated material for forming a cell casing, which material can be readily formed into a thin flat shape and the cell casing itself can flexibly be changed in shape with great latitude, so that the electric energy generating elements including a solid polymer electrolyte can be accommodated within the casing without getting damaged.
Accordingly, the effect of the invention is significant especially when the invention is applied to a sheet-like sealed non-aqueous electrolyte cell comprising a solid polymer electrolyte.
The electrolyte employed in the invention may be a solid polymer electrolyte containing LiPF6 as an electrolyte salt.
LiPF6 and LiBF4 react with moisture which was originally present in the cell or which has penetrated into the cell, to form a hydrofluoric acid or the like which could be a cause of degradation in the sealing parts of the cell casing. Where the inorganic oxide fine powder is added to the electrolyte containing LiPF6 as an electrolyte salt, the inorganic oxide fine powder can effectively adsorb the hydrofluoric acid and others. Therefore, the cell containing the inorganic oxide fine powder is less likely to undergo a decrease in the degree of sealing, compared to the prior art cells containing no inorganic oxide fine powder. In short, when the invention is applied to a sheet-like sealed non-aqueous electrolyte cell comprising a solid polymer electrolyte, containing LiPF6 as an electrolyte salt, it can more significantly exert its desirable effect.
According to the invention, the inorganic oxide fine powder may comprise alumina, magnesia, or a complex oxide composed of alumina and magnesia.
Alumina, magnesia, and complex oxides composed of alumina and magnesia have high chemical stability to the components of the electric energy generating elements such as lithium and have excellent moisture absorbency. Therefore, use of these materials as the inorganic oxide fine powder makes it possible to more reliably restrain a decrease in the degree of sealing in the sealing parts of the casing.
Referring to FIGS. 1 to 5, there will be explained a structural configuration of a non-aqueous electrolyte cell according to a preferred embodiment of the invention.
As shown in FIG. 2, the non-aqueous electrolyte cell of the invention is designed to have electric energy generating elements 6 housed within a casing 1 made from a laminated material. The casing 1 is made from a laminated material 20 having a five-layer structure as shown in FIG. 5 and is heat sealed at first to third sealing parts 2a to 2c as shown in FIG. 1 to hermetically seal the cell. It is seen from FIG. 1 that, at the first sealing part 2a, two opposed edges of the casing 1 are heat sealed with the positive and negative collector terminals 7, 8 interposed therebetween, the terminals 7, 8 being respectively attached to an anode 3 and to a cathode 4 and guided outwardly from the cell so that electric energy generated in the cell can be taken from the cell.
The electric energy generating elements 6 accommodated in the casing 1 are constituted by the anode 3 having active material layers that contain LiCoO2 as a major component and are formed on both surfaces of a positive collector; the cathode 4 having active material layers that contain a carbon material as a major component and are formed on both surfaces of a negative collector; and a separator 5 positioned between the anode 3 and the cathode 4. As the separator 5, there may be used a separator, in a narrow sense, having a function of separating the anode 3 and the cathode 4 from each other. Alternatively, an electrolytic film may be used as a separator which is composed of an electrolyte carried by a porous body of polyethylene or the like. This embodiment uses an electrolytic film composed of an electrolyte carried by a porous body and the electrolyte contains an inorganic oxide fine powder.
The structure of the laminated material 20 will be described in detail. As shown in the sectional view of FIG. 5, there is provided a 20 xcexcm-thick metal layer 1a made from aluminium. The metal layer 1a has, on the outer surface, a first polypropylene layer 1b (which is to be positioned on the periphery of the cell) having a thickness of 20 xcexcm and, on the inner surface, a second polypropylene layer 1c (which is to be positioned inside the cell) having a thickness of 60 xcexcm. The metal layer 1a and the first polypropylene layer 1b are bonded to each other with a 5 xcexcm-thick dry laminate adhesive layer 1d, whereas the metal layer 1a and the second polypropylene layer 1c are bonded to each other with a 5 xcexcm-thick carboxylic acid-denatured polypropylene layer 1e in which a carboxyl group is added to polypropylene.
The carboxylic acid-denatured polypropylene disclosed herein is made by adding a carboxylic acid to polypropylene by, for example, graft polymerization or block polymerization. Addition of a carboxylic acid promotes the heat sealing of polypropylene with respect to metal. In this embodiment (shown in FIGS. 1 to 5), carboxylic acid-denatured polypropylene is adhered to the peripheries of the collector terminals 7, 8. Specifically, there are formed carboxylic acid-denatured polypropylene layers 9 and 10 on the peripheries of the collector terminals 7, 8, which peripheries abut on the inner surface of the first sealing part 2a. By use of the collector terminals 7, 8 having such a structure, when the first sealing part 2a is subjected to heat sealing, the collector terminals 7, 8 (metal) and the second polypropylene layer 1c of the laminated material are strongly bonded to each other through the carboxylic acid-denatured polypropylene layers 9, 10, so that perfect bonding can be established at the first sealing part 2a which is most likely to lose adhesiveness. When the above structure is employed, the resultant sheet-like sealed non-aqueous electrolyte cell is less susceptible to deterioration in the degree of sealing and has high cycling life.
The sheet-like sealed non-aqueous electrolyte cell of the above structure according to the invention is fabricated in the following process.
(1) The opposed edges of the above-described laminated material of the five layer structure are overlapped (the width of the overlapped portion is 20 mm) and the overlapped edges (2b in FIG. 2) are heat sealed using a high-frequency induction heater, so as to form the laminated material into a tubular body.
(2) One opening of the tubular body is closed using the high-frequency induction heater to form a fused portion having a width of 10 mm (2c in FIG. 3). Thus, a tubular body having a closed bottom (i.e., bag-shaped cell casing) is formed in which the second sealing part 2b and the third sealing part 2c are respectively sealed.
(3) The electric energy generating elements constituted by the anode 3, cathode 4, separator 5, and electrolyte are introduced into the tubular body having a closed bottom through the opening thereof (i.e., the first sealing part 2a which has not been fused yet). At that time, the collector terminals 7, 8 attached to the anode 3 and cathode 4 are guided so as to project from the cell through the opening. It should be noted that the peripheries of the collector terminals 7, 8 are provided beforehand with the carboxylic acid-denatured polypropylene layers 9, 10 and that the anode and cathode are inserted into the casing such that the layers 9, 10 are brought into contact with the inner face of the first sealing part 2a. 
(4) Subsequently, the opposed edges of the casing material at the first sealing part 2a are heat sealed (the fused portion has a width of 10 mm) with the collector terminals 7, 8 interposed therebetween, using the high-frequency induction heater.
(5) Through the above process, the sheet-like sealed non-aqueous electrolyte cell having the laminate casing is constructed according to the invention. This cell has a width of 42 mm, length of 100 mm and thickness of 1.7 mm.
One of the important aspects of the invention resides in the use of the inorganic oxide fine powder contained in the cell, which fine powder is not an active material. While the inorganic oxide fine powder is added to the electrolyte in the above embodiment, the same desired effects can be achieved by the inorganic oxide fine powder added to other elements within the cell. More concretely, the fine powder may be present within the electric energy generating elements such as the anode, cathode and separator, or within a gap between such electric energy generating elements and the casing. It should, however, be noted that the inorganic oxide fine powder is preferably added to the electrodes and more preferably to the electrolyte. The reason for this is that the electrodes include highly reactive active materials, the electrolyte tends to entrap moisture, and the electrolyte salt, which is a component of the electrolyte, is highly reactive with moisture. Another reason is that the electrolyte has high mobility compared to other electric energy generating elements.
Alternatively, the following producing method may be employed in place of the method described earlier. A strip of the laminated material is looped with the opposed edges overlapped. After the overlapped edges (indicated by 2b in FIG. 2) are heat sealed, appropriate pressure is applied to the overlapped edges thereby to form a flat tubular body. The electric energy generating part, in which the anode and cathode are opposed with the separator interposed between, is inserted into the flat tubular body. At that time, the positive and negative collector terminals respectively attached to the anode and cathode are allowed to project outward from the tubular body. Then, the opening (2a in FIG. 1) through which the positive and negative collector terminals project outward is sealed by heat sealing with the collector terminals being interposed. Thereafter, the electrolyte is introduced through the other opening (2c) and this opening is sealed by heat sealing (see FIG. 3) thereby fabricating a finished cell.
Examples of the inorganic oxide fine powder used in the invention include alumina (aluminium oxide Al2O3), silica (silicon dioxide SiO2), zeolite, magnesia (MgO), titania (TiO2) and complex oxides produced by mixing these oxides in various combinations. Of these materials, alumina, magnesia and complex oxides composed of alumina and magnesia are preferred, particularly for a lithium cell, because they exert high absorbency to moisture and to chemical substances. They are particularly useful for the electrolyte containing LiPF6 for the following reasons.
LiPF6 reacts with water originally present in the cell and with water penetrating through the sealing parts, to produce a hydrofluoric acid which adversely affects the sealing parts (bonded parts) resulting in a decrease in the degree of sealing. This causes a vicious circle in which once the degree of sealing decreases, moisture penetrates into the cell from outside so that the yield of hydrofluoric acid increases causing more corrosion in the sealed parts. If alumina, magnesia or a complex oxide composed of alumina and magnesia exists as the inorganic oxide fine powder, it absorbs moisture and the hydrofluoric acid that is a hydrolysis product of LiPF6. Therefore, decreases in the performance of the cell and in the degree of sealing due to the presence of moisture and hydrofluoric acid can be restrained. In addition, since alumina, magnesia and complex oxides composed of alumina and magnesia enhance the viscosity of the electrolyte and reduce the scatter and vaporization of the electrolyte, there is lesser contact between the sealing parts and hydrofluoric acid etc., which consequently restrains the deterioration of the sealing parts.
The suitable amount (represented by content) of the inorganic oxide fine powder to be added to the electrolyte in the invention ranges from 0.01 to 20 wt %. If the content is below 0.01 wt %, absorbency to the hydrofluoric acid is lacking, so that the deterioration of the sealing parts cannot be satisfactorily prevented. On the other hand, if it exceeds 20 wt %, the electrolyte has excessive viscosity and the discharge rate characteristic (1C/0.2C capacity ratio) of the cell unfavourably decreases.
The average particle diameter of the inorganic oxide fine powder is preferably 20 xcexcm or less and more preferably 5 xcexcm or less. By use of the inorganic oxide fine powder having an average particle diameter of 5 xcexcm or less, enough specific surface area can be ensured to absorb hydrofluoric acid etc. to be produced. On the other hand, if it exceeds 20 xcexcm, enough specific surface area cannot be obtained so that hydrofluoric acid etc. cannot be satisfactorily absorbed.
A lower limit is not particularly specified for the particle diameter of the inorganic oxide fine powder, because the specific surface area and therefore absorbency of the powder increase as the powder becomes finer. Therefore, a lower limit for the particle diameter may be determined appropriately taking the pulverization technique etc. into account. It suffices to say that the inventors have found that desired results are obtainable with the inorganic oxide fine powder having an average particle diameter of 0.005 xcexcm.
Examples of the solvent constituting the electrolyte include organic solvents such as ethylene carbonate, vinylene carbonate and propylene carbonate, and solutions prepared by mixing these solvents. Other examples are solvents prepared by mixing any of the above solvents and low-boiling point solvents such as dimethyl carbonate, diethyl carbonate, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, and ethoxymethoxyethane.
Apart from LiPF6 which has been noted earlier, examples of the electrolyte salt to be dissolved into these solvents include LiBF4, Li(CF3SO2)2N, LiClO4 and LiCF3SO3.
As examples of the electrolyte, liquid electrolytes (electrolytic solutions), solid polymer electrolytes and gel solid polymer electrolytes are enumerated. In cases where a solid polymer electrolyte or gel solid polymer electrolyte is used, the role of a separator may be assigned to the electrolyte and such an electrolyte is the preferred element to which inorganic oxide fine powder is added.
Although the laminated material has a five-layer structure in the foregoing embodiment, it may have a different structure on condition that the structure is composed of two or more layers including at least one metal layer and one resin layer adhesive to the metal layer. However, the materials of the metal layer and the resin layer are not particularly specified. Examples of the metal foil constituting the laminated material include aluminium, steel, stainless steel and copper. Examples of the resin to be subjected to heat sealing include polyethylene, polypropylene, denatured polyethylene, denatured polypropylene, and ethylene-propylene copolymers.
While carboxylic acid-denatured polypropylene is used to form the layers 9, 10 around the collector terminals (see FIG. 4) in the foregoing embodiment, carboxylic acid-denatured polyethylene may be used in place of carboxylic acid-denatured polypropylene. Note that when carboxylic acid-denatured polyethylene is used, it is desirable in view of adhesive strength to use polyethylene as the non-denatured resin.
Although he foregoing embodiment has been discussed in the concept of the casing 1 having the first to third sealing parts 2a to 2c, the sealing parts 2b and 2c are not necessarily provided in cases where an integral casing is used. In such cases, there must be at least one sealing part (corresponding to the first sealing part 2a) in order to insert the electric energy generating elements and let the collector terminals project outwardly from the cell. Since the degree of sealing is most likely to decrease in the first sealing part. 2a through which the collector terminals project, the effect of the invention can be fully achieved even when it is applied to cells having only one sealing part.
Additionally, although the thickness of the second polypropylene layer 1c of the foregoing embodiment is 60 xcexcm, the thickness is not limited to such a value but may fall within the range of from 20 to 100 xcexcm. The thickness of the metal layer 1a is not necessarily set at 20 xcexcm as specified in the foregoing embodiment but may range from 8 to 50 xcexcm.
Finally, the active material for the anode is not limited to the materials noted above but other materials such as LiNiO2, LiMnO2 and LiFeO2 may be used.